This morning I bought a ticket, single, from Newmarket to Cheltenham.
National Rail's standard fare showed £46 for an Open ticket, via Not London. Adding in "London" as a via, showed an advance ticket for £21.50, or £28 1st class. On booking, via First Great Western's website (highly impressed with it!) there was an option "slower trains may be cheaper still - click here". Tried it, and got a booked single for £17, for a train taking 10 mins longer!
So moral of the story - cheap tickets exist, but you have to search for them. A tax on the lazy / pressed for time to research prices?
signol